Use fast_string.

opam.pins

-coq-iris https://gitlab.mpi-sws.org/FP/iris-coq 02bc52b4d905eb635940ff8545d59676ad44ea8c
+coq-iris https://gitlab.mpi-sws.org/FP/iris-coq c3dde6181d7d99007fd584d23a5dc3e859b47765

theories/lang/lang.v

 From iris.program_logic Require Export language ectx_language ectxi_language.
-From stdpp Require Export strings.
+From fast_string Require Export fast_string_lib.
 From stdpp Require Import gmap.
 Set Default Proof Using "Type".
 

theories/lifetime/lifetime_sig.v

 From iris.algebra Require Import frac.
-From stdpp Require Export gmultiset strings.
+From stdpp Require Export gmultiset.
+From fast_string Require Export fast_string_lib.
 From iris.base_logic.lib Require Export invariants.
 From iris.base_logic.lib Require Import boxes fractional.
 Set Default Proof Using "Type".